Moon’s ‘dark side’ is often confused with its far side

The headline read, “China to land on dark side of the moon.”

Unlike the Force of “Star Wars” fame, the moon has no permanent dark side. The moon goes through phases in which the lighted side consumes the darkness as the moon waxes from new to full, and then gives way as the darkness consumes the light for the second half of the month.

Much of the confusion about the moon and its phases stems from historical nomenclature. At the full moon, we see 100 percent of a full disk, but this is only one-half of the entire three-dimensional moon.

One half of the moon is always light and one half is always dark, but the side that faces toward us is not always light; likewise, the side that faces away from us is not always dark. The lighted side always faces the sun. Phases result from the changing relative positions of the sun, moon and Earth at different times of the month.

Tidal forces lock the moon such that the same side always faces Earth. That means that the moon’s rotation period is the same as its revolution. It makes one complete revolution and one complete rotation each month. The near side always faces Earth, so we see the same man-in-the-moon face all the time.

Since we only see the moon in two dimensions, it can be difficult to visualize it as a three-dimensional object.

What scientists call a quarter moon, popular jargon calls a half moon. We see half of the full moon, but we only see half of the moon’s orb. Half of a half is a quarter, so we see a quarter of the moon’s total area. At this time, a view of the far side would also show half of it illuminated.

When we see the sliver of a crescent moon, the far side is completely lit except for a tiny sliver or crescent of darkness. The same crescent occurs on the far side during a gibbous phase when almost the entire moon is lit on our near side on the nights before a full moon.

The moon’s orbit around Earth is elliptical and causes the moon’s size to vary by about 10 percent over the course of a month. The orbit is inclined from Earth’s equator by about 5 degrees, which makes it follow a path that is more difficult to predict than that of the sun, which remains on the ecliptic year-round.

The confusion about the dark side should end. It is an archaic concept, appropriate for metaphor (call out to Pink Floyd), but not for reality. Much of the time when people use dark side, what they really mean is far side.

There is a far side that is always out of our view. There is a dark side, but it changes as the moon revolves. The far side and near side are illuminated and go through phases.

Gravity keeps us stuck firmly to terra firma, holds the moon in locked orbit, and keeps the far side always out of view.

Richard Brill is a professor of science at Honolulu Community College. His column runs of the first and third Friday of the month. Email questions and comments to brill@hawaii.edu.